Model sizes don't really work in terms of MB size, but your average unit in BFME should, I estimate, be 300-700 polys. Go lower than that, by all means, but I advise only using 700+ for special units, such as Trolls or Hero units. Typically this band ends at 2000, but occasionally others, such as the Balrog, can take up to 5000. Again, these should be reserved for units of that size. Structures, on the other hand, can vary significantly. Faction buildings usually range 3000 to 10,000 (though I may be inaccurate, on the latter, I've not worked any Fortresses recently) but others such as generic walls and civilian houses should be optimized as far as possible. Preference for things such as Dale structures should be under 3000, and metropolises such as Minas Tirith should be below the 1500 mark, unless they are dedicated active (selectable) structures.
Again, these vary with examples such as the Moria modules, but that should give you an idea, I think.
Now that I'm back here, I was thinking about maybe using some of the models from other games and put them in BFME.
Don't do this because it's illegal. And misleading. And morally and socially wrong.